Multidrug resistance (MDR) is a major problem for successful chemotherapy of human cancers. One of the known mechanisms of MDR in cancer cells is the elevated expression of membrane proteins that mediate efflux of anticancer drugs. Three major membrane proteins that have this drug-efflux function have been identified: P-glycoprotein (Pgp), multidrug resistance-associated protein 1 (MRP1), and breast cancer resistance protein/mitoxantrone resistance protein (BCRP/MXR). These proteins belong to the ATP-binding cassette (ABC) membrane transporter superfamily. The long-term goal of our laboratory is to understand the molecular mechanisms of and to overcome ABC transporter-mediated MDR in cancer cells. Unlike most other ABC-transporters such as Pgp, human MRP1 has an additional membrane-spanning domain (MSD1) with a postulated extracellular amino terminus. However, our recent studies suggested that the amino terminus may be located in cytoplasm and is functionally important. In the next five years of support, we plan to test the hypothesis that the amino terminus of human MRP1 functions as a gating mechanism for drug transport and can be used as a target to inhibit MRP1 activity and to circumvent MRP1-mediated MDR. To this end, we plan to accomplish the following five specific aims: (1) to delineate the membrane orientation of the amino terminus of human MRP1;(2) to investigate the gating role of the amino terminus in drug transport function of human MRP1;(3) to determine the dimeric status and to map the dimerization domain of human MRP1;(4) to develop an inhibitor of human MRP1 from a synthetic peptide with a sequence of the amino terminus of human MRP1;and (5) to develop peptide probes of human MRP1 using synthetic peptides interacting with MSD1 of human MRP1 to investigate the functional mechanism of MRP1. The excellent scientific environment at Indiana University Cancer Research Institute and the generous institutional support will contribute enormously to the likelihood of success of this project. The information and probes obtained from this study will help us understand the molecular mechanism of human MRP1-mediated drug transport. This work may also lead us to the discovery of a new class of therapeutic agents that can help overcome drug-resistant cancers.